This invention relates generally to electronic display systems and in particular to a method and apparatus for displaying signal information with improved image details on a raster display having limited brightness levels in an electronic measurement instrument.
Oscilloscopes are a form of test and measurement instrument that has been used for many years to measure and display electrical signals as graphical waveforms. Oscilloscopes have been traditionally divided into two classes, analog and digital, based on the technology used in the instrument. The method of measuring and displaying electrical signals between these analog and digital classes of technology is very different and each technology has its own attendant advantages and disadvantages. An electrical signal to be measured is coupled to an input terminal of the measurement instrument and becomes an input signal.
Analog oscilloscopes measure and display the input signal by displacing an electron beam vertically as a function of the amplitude of the electrical signal as the beam is swept from one side of a cathode ray tube (CRT) to the other at a sweep rate. The pattern that is traced out on the phosphor of the CRT is integrated in the eyes of the oscilloscope user which allows the entire waveform to be seen. The CRT phosphor is selected to have a persistence time long enough to permit this integration over a wide range of sweep speeds.
Analog oscilloscopes tend to have a high measurement duty cycle. Measurement duty cycle is the ratio of measurement time to the time spent processing the measurement and is also known as "dead time." Most of the dead time of the analog oscilloscope is the time during which the electron beam is returned from the end of a sweep back to the beginning to start another sweep. It is desirable to have a high measurement duty cycle in order to display as much measurement information as possible because any information from the input signal that arrives during the dead time is lost. At higher sweep rates, the display of the analog oscilloscope is updated very rapidly, thereby giving the displayed waveform a "live" look that provides the ability to reveal a great deal of information about the input signal to the user, particularly in situations involving complex input signals which have considerable variation over a given measurement time. The ability to display a large amount of information about the input signal with a high measurement duty cycle tends to make the analog oscilloscope a good qualitative tool.
Digital storage oscilloscopes, (DSO's), operate by digitizing the input signal into discrete digital samples using an analog to digital converter (ADC), storing the digital samples in trace memory, and then converting the digital samples into graphical traces in a trace memory for subsequent display as a graphical image, typically on a raster display. A raster display typically uses a two dimensional array or matrix of picture elements (pixels) arranged in rows and columns, with each pixel assigned a brightness value. A typical raster display has hundreds of rows and columns with which to construct a display image with the raster display update rate being independent of the sweep or measurement rate.
DSO's have an advantage over analog oscilloscopes by having the ability to store, recall, and perform calculations on the stored digital samples. As such, DSO's tend to be good quantitative tools for accurately measuring voltage and time characteristics of the input signal. However, the measurement duty cycle of DSO's, along with the update rate, tends to be substantially lower than that of analog oscilloscopes. In a DSO, the full measurement speed is only available when the trace memory is being filled while the remaining time is dead time in which the digital samples are processed. The DSO, therefore, while a good quantitative tool, tends to be not as good for use as a qualitative tool for visualizing the real time behavior of the input signal because of its relatively low measurement duty cycle.
A single acquisition of the input signal over a predetermined acquisition time captures a view of the electrical signal in the spatial domain, which is the amplitude variation of the input signal versus acquisition time. A complex electrical signal may have variations over multiple acquisitions in the temporal domain which is amplitude variation versus acquisition number. Thus, the changing character of the electrical signal in the temporal domain may be captured over multiple acquisitions which are stored as the display image. The display image is thus a function of acquisition time, which remains constant, and temporal time, which is as long as desired and measured in the terms of the number of acquisitions.
A new DSO measurement architecture has recently emerged that substantially improves the update rate and measurement duty cycle. In this architecture, digital samples are converted directly to pixel information as quickly as they arrive from the ADC to build a display image over numerous acquisitions. The display image, because of its increased measurement duty cycle, more closely resembles the traditional analog oscilloscope in providing a qualitative image of the electrical signal.
However, because the display image is stored only in the form of pixel information and not in terms of actual measurement values, the qualitative value of the display image is reduced because precise measurements are no longer possible. Because the multiple acquisitions must be mapped onto a limited set of pixels in the raster display with each pixel having a limited set of brightness levels, various methods have been devised for building the display image while preserving the temporal information over multiple acquisitions of the electrical signal. Prior art DSO's typically use false color to represent the frequency of which any particular pixel has been accessed over multiple acquisitions. In this way, transient signals and infrequent anomalies may be highlighted with a different color from that of a repetitious signal having substantially the same temporal behavior over multiple acquisitions.
DSO's are increasingly being designed as portable, handheld, battery-operated instruments, often using liquid crystal display (LCD) technology. Readily-available LCD technology effectively provides only four levels of brightness, typically in a monochrome LCD display. Power consumption, physical size, and LCD display brightness and resolution are critical factors in the design of a portable DSO. At the same time, there is a substantial need for DSO's to adopt the advantages of their analog oscilloscope counterparts in having relatively high update rates and the ability to perceive structural details of complex input signals with high variation in the temporal domain. It would be therefore be desirable to provide an improved method of displaying the temporal domain image of an electrical signal with enhanced image details using a limited number of brightness levels for each pixel using adaptive processes that select allocate the pixels to each of the brightness levels by predetermined portions within each column of each sweep. It would be further desirable that the present brightness levels be adaptively combined with past brightness levels to effectively display a composite signal based on both the present and past signal characteristics.